Multiple zone induction coil power control apparatus and method

ABSTRACT

A plurality of zones of an induction heating coil are individually controlled by varying the current through one or more zones of the coil to obtain a desired temperature profile in a workpiece. The current flow through a zone of the coil is determined by the conduction state of an associated saturable reactor, which is controlled in accordance with a preselected value or a variable value generated, for example, by a computer.

BACKGROUND OF THE INVENTION

The induction heating of metal products to desired temperatures iswell-known and commonly practiced. In conventional induction heating, ametal workpiece is heated by an induction heating coil by placing thecoil around the workpiece and passing electric current through the coil.The electric current passing through the coil produces a magnetic fieldand induces secondary currents in the workpiece. The secondary currentsflowing through the workpiece heat it.

It is sometimes desirable to heat different areas or zones of theworkpiece so as to obtain a non-uniform temperature profile along thelength of the workpiece. By applying different amounts of power todifferent zones of a workpiece placed within the induction coil,reproducible temperature profiles can be obtained. These reproducibletemperature profiles yield desirable effects in the workpiece,especially in metallurgical processes involving crystal growth.

In accordance with the present invention, a desired temperature profileis obtained by shunting various zones of the induction heating coil,corresponding to various zones of a workpiece, with a saturable reactor.For a particular combination of voltage and current through a zone ofthe heating coil, the saturable reactor may be made to conduct anddivert current from the zone of the heating coil. By controlling theamount of current diverted, or shunted, across a zone, the power in thatzone, and therefore the temperature of the workpiece in that zone, maybe controlled.

It is an object of the present invention to individually control theamount of power to one or more of several zones of an induction heatingcoil to produce a desired temperature profile in a workpiece.

SUMMARY OF THE INVENTION

The present invention is an apparatus for individually controlling powerdelivered to each of a plurality of zones of an induction heating coilso as to provide a desired temperature profile in a workpiece heated bythe coil. The apparatus comprises a high-frequency induction powersupply for delivering power to the coil and means for measuring thepower in each zone. The apparatus also comprises means for comparing thepower in each zone to a predetermined reference and generating a firstcontrol signal based on the comparison and means operatively associatedwith each zone in response to the first control signal for divertingelectric current around that zone to thereby control the power deliveredto the zone. The apparatus further comprises means for determining thetotal power delivered by the power supply, means for adding the power ineach zone to determine the total power in all zones, and means forcomparing the total power in all zones to the total power delivered bythe power supply and generating a second control signal based on thecomparison for controlling the total power delivered by the powersupply.

The present invention also includes a method for individuallycontrolling the power delivered to each of a plurality of zones of aninduction heating coil so as to provide a desired temperature profile ina workpiece heated by the coil. The method comprises the steps ofdelivering high frequency power to the coil, measuring the power in eachzone of the coil, comparing the power in each zone to a predeterminedreference and generating a first control signal based on the comparison,diverting electric current around that zone in response to the firstcontrol signal to thereby control the power delivered to that zone,determining the total power delivered to the coil, adding the power ineach zone to determine the total power in all zones to the total powerdelivered to the coil and generating a second control signal based onthe comparison for controlling the total power delivered to the coil.

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a control apparatus in accordance withthe present invention.

FIG. 2(a) is a curve showing the relationship between control currentand load current in a saturable reactor.

FIG. 2(b) is a curve showing the relationship between control current ina saturable reactor and heating coil current controlled by the saturablereactor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a schematic diagram of a controlcircuit in accordance with the present invention, generally designatedby the numeral 10.

A high-frequency induction power supply 12 generates a high-frequency acvoltage. Power supply 12 may be manually adjustable to deliver a desiredpower output, and is preferably a constant current power supply. In theembodiment shown in the drawing, power supply 12 includes an inverterstage having two silicon controlled rectifiers (SCRs) or thyristors 14and 16 connected in series between a positive voltage source B+ and anegative voltage source B-. The output current of power supply 12 iscontrolled by SCRs 14 and 16, as will be more fully explained below. Themanner in which SCRs 14 and 16 may be switched, or "gated", and theiroperation in current-limiting power supplies will be well understood bythose skilled in the art, and need not be described here in detail.

The cathode of SCR 14 and the anode of SCR 16 are connected together atnode 17, which represents the output terminal of power supply 12. Node17 is connected to one terminal of capacitor 18. The opposite terminalof capacitor 18 is connected to one terminal, or leg, of the primarywinding of load matching transformer 20. The other leg of the primary oftransformer 20 is connected to a neutral potential. The secondarywinding of transformer 20 is connected essentially in series with theinduction heating coil which is composed of coil sections 38, 40 and 42,which are connected in series at nodes 39 and 41. Capacitor 22 andreactor 24 are inserted in series with one leg of the secondary oftransformer 20 between the transformer and the heating coil. Capacitors18 and 22 provide power factor correction to maximize power transferfrom the power supply 12 to the induction heating coil sections 38, 40and 42, and also serve to determine the resonant frequency of the loadcircuit. Reactor 24 is a stabilizing reactor which eliminates doublefrequency harmonics introduced when the saturable reactors areconducting. Reactor 24 preferably has about three times the inductanceof the heating coil sections 38, 40 and 42, so that any variation inheating coil impedance during operation will have only a small effect onthe impedance of the load circuit. The operation of the saturablereactors and their effect will be explained more fully below.

As noted above, the induction heating coil is composed of three coilsections 38, 40 and 42, although any number of coil sections may be usedwithout departing from the scope of the present invention. However,three coil sections suffice to explain the invention. Each coil section38, 40 and 42 defines a zone, in this case zone 1, zone 2 and zone 3,respectively, of the workpiece W.

A saturable reactor 26, 28 and 30 is placed across (i.e., in parallelwith) each of the coil sections 38, 40 and 42, respectively. Eachsaturable reactor is connected with its secondary winding in parallelwith its associated coil section so as to divert, or shunt, currentaround the associated coil section. Each saturable reactor 26, 28 and 30contains a saturable element or core 32, 34 and 36, respectively, ofhigh magnetic permeability.

The saturable reactors control the amount of current through theassociated section of the heating coil. The primary, or control, windingof each reactor carries a direct current, called the control current, ofadjustable magnitude, which can saturate the core. The dc current isprovided by power transducers and comparators 56, 58 and 60, as will beexplained more fully below. The magnitude of the control currentdetermines the extent to which the core is saturated. The intensity ofsaturation of the core in turn controls the effective inductance of thesecondary, or load, winding of the reactor. As will be understood bythose skilled in the art, the relationship between control current andthe inductance of the load winding has a linear range between the pointswhere the core is fully saturated. See FIG. 2(a). Since the impedance ofthe load winding at a given frequency is proportional to the inductance,the relationship between the load winding impedance and the controlcurrent is also linear in the range between the extremes of saturation.Naturally, since load current is proportional to the impedance of theload winding, the relationship between the control current and the loadcurrent also has a linear range.

When the core is fully saturated by the control current, the effectiveinductance (and therefore the impedance) of the load winding is small.Reducing the magnitude of the control current reduces the intensity ofsaturation of the core. This increases the impedance of the load windingand brings the reactor into the linear range of operation. Thus, bycontrolling the dc voltage applied across the control winding of thereactor, the impedance of the load winding of the reactor may becontrolled. When the voltage across the control winding is such that theload winding has a very high impedance, virtually no current will flowthrough the load winding. In this case, all current will flow throughthe associated coil section. Conversely, when the voltage across thecontrol winding is such that the impedance of the load winding is low,current will flow through the load winding instead of the associatedcoil section, thus shunting current around the associated coil section.In between these extremes, in the linear range, the current through theload winding is proportional to the control current.

As will be appreciated, when the impedance of the load winding is low,no in phase current flows through the associated coil section, andtherefore the power delivered by that coil section to the workpiece iszero. Conversely, when the impedance of the load winding is high, all ofthe current flows through the associated coil section, and thus thepower delivered by the coil section is at its maximum. For pointsbetween these extremes, current in the coil section is inverselyproportional to the control current and varies linearly. See FIG. 2(b).It can thus be seen that varying the impedance of the load winding ofone of saturable reactors 26, 28 or 30 varies the power delivered by theassociated coil section 38, 40 or 42 to the workpiece.

A side effect of the operation of the saturable reactors 26, 28 and 30is the introduction of double frequency harmonics. When one of thesaturable reactors is conducting, it will conduct current during aportion of both the positive and negative swings of the current in thesecondary of load matching transformer 20, thereby introducing thedouble harmonic frequency component. Stabilizing reactor 24 is placed inseries with the secondary of transformer 20 to eliminate the doublefrequency harmonic component.

Power in each coil section 38, 40 and 42 is sensed by potentialtransformers 44, 46 and 48 and current transformers 50, 52 and 54respectively. Potential transformer 44 and current transformer 50provide the inputs to power transducer and comparator 56, potentialtransformer 46 and current transformer 52 provide the inputs to powertransducer and comparator 58, and potential transformer 48 and currenttransformer 54 provide the inputs to power transducer and comparator 60.Power transducers and comparators 56, 58 and 60 compute the power incoil sections 38, 40 and 42, respectively, based on the voltage at thesecondary of the potential transformer 44, 46 and 48, respectively, andthe current sensed by the current transformer 50, 52 and 54,respectively. The product of the sensed voltage and sensed currentyields the sensed power in the associated coil section.

The sensed power is compared within power transducers and comparators56, 58 and 60 to a predetermined set point, or reference, power. Theoutputs of power transducers and comparators 56, 58 and 60 will be a dcvoltage proportional to the difference between the sensed and referencepowers. The outputs of power transducers annd comparators 56, 58 and 60provide the control currents to the control windings of saturablereactors 26, 28 and 30, respectively. Accordingly, the intensity ofsaturation of the core of the associated saturable reactor 26, 28 and 30is varied in response to the dc output of comparators 56, 58 and 60,respectively, so as to increase or decrease the load impedance of thereactor, and thus the current shunted around the associated coilsection.

The outputs of power transducers and comparators 56, 58 and 60 are alsosummed in power adder 62. The output of power adder 62 thus representsthe total power being dissipated in coil sections 38, 40 and 42. Theoutput of power adder 62 provides one input to the current and powercontrol circuit 68. The second input to current and power controlcircuit 68 is the output of current transducer 66. The input of currenttransducer 66 is derived from current transformer 64, which is locatedin the return leg of the secondary of load matching transformer 20.Since current transformer 64 is located in series with the secondary ofload matching transformer 20, current transformer 64 senses the totalcurrent in the secondary of load matching transformer 20. That is,current transformer 64 senses not only current flowing through coilsections 38, 40 and 42, but current shunted by saturable reactors 26, 28and 30 as well. The current sensed by current transformer 64 isproportional to, and thus a measure of, the total power supplied to theload circuit by the secondary of load matching transformer 20.

Current and power control circuit 68 may be any conventional analogcomparison circuit and compares the total power being supplied by thesecondary of load matching transformer 20 to the desired output. Basedon this comparison, current and power control circuit 68 generatesgating pulses which control the gating of SCRs 14 and 16. The frequencyof the gating pulses is increased or decreased depending upon whethermore of less current is required from power supply 12. Changing thefrequency of the gating pulses changes the frequency of the power supplyoutput. It is known that for a given set of conditions, the load circuitof transformer 20 will have a resonant frequency. Current, and hencepower, to the load circuit will be at a maximum when the frequency ofpower supply 12 is at that resonant frequency. Current, annd hencepower, to the load circuit will decrease as the frequency of powersupply 12 decreases from resonance. Thus, by controlling the firing rateof SCRs 14 and 16, the total current delivered by the secondary of loadmatching transformer 20, and hence the total power, can be controlled.

The output of power adder 62 is compared in current and power controlcircuit 68 to a maximum power reference which represents the maximumpower which can safely be drawn from power supply 12. Any conventionalcomparison circuitry may be used. Current and power control circuit 68limits in known manner the output current of power supply 12 based onthe comparison so that the power output of power supply 12 will notexceed a safe maximum.

The power delivered to coil sections 38, 40 and 42 may thus be variedaccording to any desired temperature profile to achieve the desiredresults in workpiece W. The precise details of current and power controlcircuit 68 and comparators 56, 58 and 60 are not crucial to the presentinvention. Any convenient and conventional control and comparatorcircuitry may be employed without departing from the scope of thepresent invention.

The desired temperature profile likewise may be generated in anyconvenient and conventional manner, and may be a predetermined profileor a variable profile generated, for example, by a computer.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

We claim:
 1. Induction heating apparatus for providing a desiredtemperature profile in a workpiece to be heated, comprising:(a) aninduction heating coil having a plurality of zones, (b) a high-frequencyinduction power supply for delivering power to the coil, (c) controlmeans for individually controlling the power delivered to each zone ofthe coil, the control means comprising:(i) means for measuring the powerin each zone, (ii) means for comparing the power in each zone to apredetermined reference and generating a first control signal based onthe comparison for each respective zone, (iii) means operativelyassociated with each zone and responsive to the first control signalassociated with the respective zone for diverting electric currentaround the respective zone to thereby control the power delivered to therespective zone, (iv) means for determining total power delivered by thepower supply, (v) means for adding the power in each zone to determine atotal power in all zones, and (vi) means for comparing the total powerin all zones to the total power delivered by the power supply andgenerating a second control signal based on the comparison forcontrolling the total power delivered by the power supply.
 2. Apparatusaccording to claim 1, wherein the high-frequency induction power supplyis directly responsive to the second control signal.
 3. Apparatus as inclaim 2, wherein the power supply includes at least two switch meansconnected in series between a positive voltage source and a negativevoltage source, each switch means being controlled by the second controlsignal.
 4. Apparatus as in claim 3, wherein the switch means are siliconcontrolled rectifiers.
 5. Apparatus according to claim 1, wherein themeans for measuring the power in each zone includes means for sensingthe potential across the portion of the coil in the respective zone andmeans for sensing the current through the portion of the coil in therespective zone.
 6. Apparatus according to claim 5, wherein the meansfor sensing the potential is a potential transformer.
 7. Apparatusaccording to claim 5, wherein the means for sensing the current is acurrent transformer.
 8. Apparatus as in claim 1, wherein the meansoperatively associated with each zone and responsive to the firstcontrol signal associated with the respective zone for divertingelectric current around the respective zone is a saturable reactorconnected in parallel with the portion of the coil in the respectivezone.
 9. Apparatus as in claim 1, wherein the means for determiningtotal power delivered by the power supply includes means for sensingtotal current delivered by the power supply.
 10. Apparatus as in claim9, wherein the means for sensing total current is a current transformer.11. Induction heating apparatus for providing a desired temperatureprofile in a workpiece to be heated, comprising:(a) an induction heatingcoil having a plurality of zones, (b) a high frequency induction powersupply for delivering power to the coil, (c) control means forindividually controlling the power delivered to each zone of the coil,the control means comprising:(i) means for sensing the voltage andcurrent in each zone, (ii) means for computing the power in each zonefrom the sensed voltage and sensed current in the respective zone, (iii)means for comparing the power in each zone to a predetermined referenceand generating a first control signal based on the comparison for eachrespective zone, (iv) shunt means connected in parallel with the coilsection in each zone and responsive to the first control signalassociated with the respective zone for shunting electric current aroundthe respective zone to thereby control the power delivered to therespective zone, (v) means for sensing total current delivered by thepower supply, (vi) means for calculating from the sensed total currentthe total power delivered by the power supply, (vii) means for addingthe power in each zone to determine a total power in all zones, and(viii) means for comparing the total power in all zones to the totalpower delivered by the power supply and generating a second controlsignal based on the comparison to limit the output of the power supplyto a predetermined maximum.
 12. Apparatus as in claim 11, wherein theshunt means is a saturable reactor, the control winding of which iscontrolled by the first control signal associated with the respectivezone and the load winding of which is connected in parallel with thecoil section in the respective zone.
 13. Induction heating apparatus forproviding a desired temperature profile in a workpiece to be heated,comprising:(a) an induction heating coil having a plurality of zones,(b) a high-frequency induction power supply for delivering power to thecoil, the power supply having at least two switch means connected inseries between a positive voltage source and a negative voltage source,the output of the power supply being controllable in response to theconduction state of the switch means, (c) control means for individuallycontrolling the power delivered to each zone of the coil, the controlmeans comprising:(i) means for sensing the voltage and current in eachzone, (ii) means for computing the power in each zone from the sensedvoltage and sensed current in the respective zone, (iii) means forcomparing the power in each zone to a predetermined reference andgenerating a first control signal based on the comparison for eachrespective zone, (iv) a saturable reactor operatively associated witheach zone and having its load winding connected in parallel with thecoil section in the respective zone for shunting electric current aroundthe respective zone to thereby control the power delivered to therespective zone, the control winding of the saturable reactor beingcontrolled by the first control signal associated with the respectivezone, (v) means for sensing total current delivered by the power supply,(vi) means for calculating from the sensed total current the total powerdelivered by the power supply, (vii) means for adding the power in eachzone to determine a total power in all zones, and (viii) means forcomparing the total power in all zones to the total power delivered bythe power supply and generating a second control signal based on thecomparison to control the conduction state of the switch means in thepower supply.
 14. Method for individually controlling the powerdelivered to each of a plurality of zones of an induction heating coilso as to provide a desired temperature profile in a workpiece heated bythe coil, comprising the steps of:(a) delivering high frequency power tothe coil, (b) measuring the power in each zone of the coil, (c)comparing the power in each zone to a predetermined reference andgenerating a first control signal based on the comparison for eachrespective zone, (d) diverting electric current around each zone inresponse to the first control signal associated with the respective zoneto thereby control the power delivered to the respective zone, (e)determining the total power delivered to the coil, (f) adding the powerin each zone to determine the total power in all zones, and (g)comparing the total power in all zones to the total power delivered tothe coil and generating a second control signal based on the comparisonfor controlling the total power delivered to the coil.
 15. Method forindividually controlling the power delivered to each of a plurality ofzones of an induction heating coil so as to provide a desiredtemperature profile in a workpiece heated by the coil, comprising thesteps of:(a) delivering variable magnitude high frequency power to thecoil, (b) sensing the voltage and current in each zone of the coil, (c)computing the power in each zone from the sensed voltage and sensedcurrent in the respective zone, (d) comparing the power in each zone toa predetermined reference and generating a first control signal based onthe comparison for each respective zone, (e) shunting current aroundeach zone in a path connected electrically in parallel with the coilsection in the respective zone in response to the first control signalassociated with the respective zone to thereby control the powerdelivered to the respective zone, (f) sensing total current delivered bythe power supply, (g) calculating from the sensed total current to thetotal power delivered by the power supply, (h) adding the power in eachzone to determine a total power in all zones, and (i) comparing thetotal power in all zones to the total power delivered by the powersupply and generating a second control signal based on the comparison tolimit the high frequency power delivered to the coil to a predeterminedmaximum.